Activation of brainstem endorphinergic neurons causes cardiovascular depression and facilitates baroreflex bradycardia

Activation of hypothalamic AgRP and POMC neurons evokes disparate sympathetic and cardiovascular responses

The arcuate nucleus of the hypothalamus (ARC) plays a key role in linking peripheral metabolic status to the brain melanocortin system, which influences a wide range of physiological processes including the sympathetic nervous system and blood pressure. The importance of the activity of agouti-related peptide (AgRP)- and proopiomelanocortin (POMC)-expressing neurons, two molecularly distinct populations of ARC neurons, for metabolic regulation is well established, but their relevance for sympathetic and cardiovascular control remains unclear. We used designer receptors exclusively activated by designer drug (DREADD) technology to study how activation of AgRP and POMC neurons affect renal sympathetic nerve traffic and blood pressure. In addition to the drastic feeding-stimulatory effect, DREADD-mediated activation of AgRP, but not POMC neurons, induced an acute reduction in renal sympathetic nerve activity in conscious mice. Paradoxically, however, DREADD-mediated chronic activation of AgRP neurons caused a significant increase in blood pressure specifically in the inactive light phase. On the other hand, chronic activation of POMC neurons led to a significant reduction in blood pressure. These results bring new insights to a previously unappreciated role of ARC AgRP and POMC neuronal activity in autonomic and cardiovascular regulation.NEW & NOTEWORTHY Agouti-related peptide (AgRP)- and proopiomelanocortin (POMC)-expressing neurons of the arcuate nucleus are essential components of the brain melanocortin system that controls various physiological processes. Here, we tested the metabolic and cardiovascular effects of direct activation of these two populations of neurons. Our findings show that, in addition to stimulation of food intake, chemogenetic mediated activation of hypothalamic arcuate nucleus AgRP, but not POMC, neurons reduce renal sympathetic traffic. Despite this, chronic activation of AgRP neurons increased blood pressure. However, chronic activation of POMC neurons led to a significant reduction in blood pressure. Our findings highlight the importance of arcuate nucleus AgRP and POMC neuronal activity in autonomic and cardiovascular regulation.

Role of the hypothalamic arcuate nucleus in cardiovascular regulation

Recently the hypothalamic arcuate nucleus (Arc) has been implicated in cardiovascular regulation. Both pressor and depressor responses can be elicited by the chemical stimulation of the Arc. The direction of cardiovascular responses (increase or decrease) elicited from the Arc depends on the baseline blood pressure. The pressor responses are mediated via increase in sympathetic nerve activity and involve activation of the spinal ionotropic glutamate receptors. Arc-stimulation elicits tachycardic responses which are mediated via inhibition of vagal input and excitation of sympathetic input to the heart. The pathways within the brain mediating the pressor and tachycardic responses elicited from the Arc have not been delineated. The depressor responses to the Arc-stimulation are mediated via the hypothalamic paraventricular nucleus (PVN). Gamma aminobutyric acid type A receptors, neuropeptide Y1 receptors, and opiate receptors in the PVN mediate the depressor responses elicited from the Arc. Some circulating hormones (e.g., leptin and insulin) may reach the Arc via the leaky blood-brain barrier and elicit their cardiovascular effects. Although the Arc is involved in mediating the cardiovascular responses to intravenously injected angiotensin II and angiotensin-(1-12), these effects may not be due to leakage of these peptides across the blood-brain barrier in the Arc; instead, circulating angiotensins may act on neurons in the SFO and mediate cardiovascular actions via the projections of SFO neurons to the Arc. Cardiovascular responses elicited by acupuncture have been reported to be mediated by direct and indirect projections of the Arc to the RVLM.

An arcuate-ventrolateral periaqueductal gray reciprocal circuit participates in electroacupuncture cardiovascular inhibition

Electroacupuncture (EA) suppresses elevated blood pressure (BP) by activating the arcuate nucleus, ventrolateral periaqueductal gray (vlPAG), and inhibiting cardiovascular sympathetic neurons in the rostral ventrolateral medulla. This study investigated the reciprocal neural circuit between arcuate and vlPAG during EA inhibition of reflex increases in blood pressure. In α-chloralose anesthetized cats the gallbladder or splanchnic nerve was stimulated to induce cardiovascular sympathoexcitatory reflexes. Electrophysiological recordings showed that EA facilitates the arcuate neural response to splanchnic nerve stimulation. Bilateral vlPAG microinjection of D,L-homocysteic acid (DLH) facilitated the arcuate response to splanchnic nerve stimulation, while microinjection of kainic acid blocked EA (P 5-6 acupoints on pericardial meridian, overlying the median nerves) excitation of arcuate neurons. Retrograde microsphere tracer labeling in the arcuate or vlPAG perikarya was found after respective microinjection of the tracer in the vlPAG or arcuate of rats, demonstrating reciprocal direct connections between the two nuclei. EA inhibition of reflex-induced BP elevation was blocked by injection of glutamate or cholinergic receptor antagonist, kynurenic acid or atropine, into the arcuate. Excitation of vlPAG neurons during EA was blocked by arcuate microinjection of glutamate NMDA and non-NMDA receptor antagonists, AP-5 and CNQX, or the cholinergic receptor antagonist, atropine. Microinjection of DLH or acetylcholine (ACh) into the arcuate facilitated EA excitation of vlPAG neurons. Microinjection of AP5 and CNQX, but not atropine, into the vlPAG blocked EA excitation of arcuate neurons. Thus, a reciprocal excitatory glutamatergic neural circuit between the arcuate and vlPAG contributes to long-lasting EA cardiovascular inhibition. ACh in the arcuate but not in the vlPAG participates in the reciprocal excitation.

β-Endorphin involvement in the regulatory response to body sodium overload

The present study was performed to examine the role of the endogenous beta-endorphinergic system on blood pressure regulation, sympathetic and brain activity during body sodium overload. Beta-endorphin knockout (beta end-/-), heterozygous (beta end+/-) and wild-type (beta end+/+) mice were submitted for two weeks to either a normal- or a high-sodium diet (NSD and HSD, respectively), and systolic blood pressure (SBP), urinary catecholamines (as an index of sympathetic nervous system activity), and the brain pattern of Fos-like immunoreactivity (as a marker of neuronal activation) were evaluated in each group. HSD caused a significant increase in SBP in beta end-/- mutant mice compared with beta end+/+ mice kept in the same experimental conditions (P < 0.01), but no statistical differences were observed between beta end+/- and beta end+/+ on a HSD. Moreover, when animals from the three genetic lines were fed with a NSD no changes in SBP were evidenced. With regard to brain activity, beta end-/- mice maintained on a HSD showed a significant increase in Fos-like immunoreactive neurons in the median preoptic nucleus (P < 0.01) compared with beta end+/- and beta end+/+ animals. Additionally, beta end-/- mice had higher levels of urinary epinephrine excretion (P < 0.05) on a HSD in comparison to beta end+/+ and beta end+/- animals in the same experimental conditions. No differences, however, were registered in norepinephrine and dopamine urinary excretion in animals from the three genetic lines after two weeks on either a HSD or a NSD. In summary, our results indicate that the beta-endorphinergic system may play a part in the compensatory response to sodium overload, since the absence of beta-endorphin causes an increase in systolic blood pressure, and increases median preoptic nucleus neural activity and urinary epinephrine excretion.

Cardiovascular actions of adrenocorticotropin microinjections into the nucleus tractus solitarius of the rat

The presence of adrenocorticotropin (ACTH) containing cells and melanocortin (MC) receptors has been reported in the nucleus tractus solitarius (NTS) of the rat. The importance of the NTS in the regulation of cardiovascular function is also well established. Based on these reports, it was hypothesized that ACTH acting within the NTS may modulate the central regulation of cardiovascular function. To test this hypothesis, cardiovascular effects of ACTH in the NTS were investigated in intact urethane-anesthetized and unanesthetized decerebrate, artificially ventilated, adult male Wistar rats. Microinjections of ACTH (0, 0.5, 1, 2, and 4 mM) into the medial subnucleus of NTS (mNTS) elicited decreases in mean arterial pressure (MAP; 0+/-0, 24.4+/-3.5, 35.7+/-4.3, 44.5+/-5.8 and 53.7+/-5.6 mm Hg, respectively) and heart rate (HR; 0+/-0, 25.7+/-5.3, 35.5+/-6.4, 47.5+/-12.1 and 55.0+/-5.6 beats/min, respectively). The onset and duration of the responses to microinjections of ACTH (0.5-4 mM) were 5-10 s and 45-120 s, respectively. Control microinjections of artificial cerebrospinal fluid (aCSF) did not elicit any response. The volume of all microinjections was 100 nl. The concentrations of ACTH that elicited depressor and bradycardic responses when microinjected into the mNTS (e.g. 1 or 2 mM, 100 nl), did not elicit a response when injected i.v. (n=5) or i.c.v. (n=2) indicating that there was no leakage of the drug from the injection site in the mNTS. Microinjections of MC3/4 receptor antagonists (acetyl-[Nle(4), Asp(5), d-2-Nal(7), Lys(10)]-cyclo-alpha-MSH amide, fragments 4-10 (SHU9119) and agouti-related protein (83-132) amide) into the mNTS blocked the responses to ACTH. Microinjections of ACTH (2 mM) into the mNTS decreased efferent greater splanchnic nerve activity. Bilateral vagotomy significantly attenuated ACTH-induced bradycardia. These results indicated that: 1) microinjections of ACTH into the mNTS elicited depressor and bradycardic responses, 2) these responses were mediated via MC3/4 receptors, 3) the depressor effects were mediated via a decrease in the activity of the sympathetic nervous system, and 4) the bradycardic responses were vagally mediated.

Opioids and opiates: analgesia with cardiovascular, haemodynamic and immune implications in critical illness

Traumatic injury, surgical interventions and sepsis are amongst some of the clinical conditions that result in marked activation of neuroendocrine and opiate responses aimed at restoring haemodynamic and metabolic homeostasis. The central activation of the neuroendocrine and opiate systems, known collectively as the stress response, is elicited by diverse physical stressor conditions, including ischaemia, glucopenia and inflammation. The role of the hypothalamic-pituitary-adrenal axis and sympathetic nervous system in counterregulation of haemodynamic and metabolic alterations has been studied extensively. However, that of the endogenous opiates/opioid system is still unclear. In addition to activation of the opiate receptor through the endogenous release of opioids, pharmacotherapy with opiate receptor agonists is frequently used for sedation and analgesia of injured, septic and critically ill patients. How this affects the haemodynamic, cardiovascular, metabolic and immune responses is poorly understood. The variety of opiate receptor types, their specificity and ubiquitous location both in the central nervous system and in the periphery adds additional complicating factors to the clear understanding of their contribution to the stress response to the various physical perturbations. This review aims at discussing scientific evidence gathered from preclinical studies on the role of endogenous opioids as well as those administered as pharmacological agents on the host cardiovascular, neuroendocrine, metabolic and immune response mechanisms critical for survival from injury in perspective with clinical observations that provide parallel assessment of relevant outcome measures. When possible, the clinical relevance and corresponding scenarios where this evidence can be integrated into our understanding of the clinical implications of opiate effects will be examined. Overall, the scientific basis to enhance clinical judgment and expectations when using opioid sedation and analgesia in the management of the injured, septic or postsurgical patient will be discussed.

Hemorrhage activates proopiomelanocortin neurons in the rat hypothalamus

Severe blood loss lowers arterial pressure through a central mechanism that is thought to include opioid neurons. In this study, we investigated whether hemorrhage activates proopiomelanocortin (POMC) neurons by measuring Fos immunoreactivity and POMC mRNA levels in the medial basal hypothalamus. Hemorrhage (2.2 ml/100 g body weight over 20 min) increased the number of Fos immunoreactive neurons throughout the rostral-caudal extent of the arcuate nucleus, the retrochiasmatic area and the peri-arcuate region lateral to the arcuate nucleus where POMC neurons are located. Double label immunohistochemistry revealed that hemorrhage increased Fos expression by beta-endorphin immunoreactive neurons significantly. The proportion of beta-endorphin immunoreactive neurons that expressed Fos immunoreactivity increased approximately four-fold, from 11.7+/-1.4% in sham-operated control animals to 42.0+/-5.2% in hemorrhaged animals. Hemorrhage also increased POMC mRNA levels in the medial basal hypothalamus significantly, consistent with the hypothesis that blood loss activates POMC neurons. To test whether activation of arcuate neurons contributes to the fall in arterial pressure evoked by hemorrhage, we inhibited neuronal activity in the caudal arcuate nucleus by microinjecting the local anesthetic lidocaine (2%; 0.1 or 0.3 microl) bilaterally 2 min before hemorrhage was initiated. Lidocaine injection inhibited hemorrhagic hypotension and bradycardia significantly although it did not influence arterial pressure or heart rate in non-hemorrhaged rats. These results demonstrate that hemorrhage activates POMC neurons and provide evidence that activation of neurons in the arcuate nucleus plays an important role in the hemodynamic response to hemorrhage.

Carotid baroreflex in the rat: role of glutamate receptors in the medial subnucleus of the solitary tract

Experiments were done in urethane-anesthetized adult male Wistar rats to investigate the role of glutamate receptors in the medial subnucleus of the solitary tract (mNTS) in mediating the carotid sinus baroreflex responses. The carotid sinus on one side was isolated from the general circulation and perfused with a warm perfusion fluid (37 degrees C; pH 7.4) saturated with 100% oxygen. The carotid sinus was then connected to an apparatus that permitted application of pressure increments (20-100 mm Hg) to stimulate specifically baroreceptors. The mNTS ipsilateral to the isolated carotid sinus was identified by microinjections (100 nL) of L-glutamate (5 mM). The stereotaxic coordinates for mNTS were: 0.5-0.6 mm rostral to the calamus scriptorius, 0.5-0.6 mm lateral to the midline, and 0.5-0.6 mm deep from the dorsal medullary surface. Microinjections of either D(-)-2-amino-7-phosphono-heptanoic acid, which is an N-methyl-D-aspartic acid (NMDA) receptor antagonist (5 mM) or 2,3-dioxo-6-nitro-1,2,3,4-tetrahydro-benzo[f]quinoxaline-7-sulfonamide disodium (a non-NMDA receptor antagonist; 2 mM) significantly attenuated the depressor responses elicited by carotid baroreceptor stimulation. Simultaneous blockade of NMDA and non-NMDA receptors in the ipsilateral mNTS completely abolished the depressor responses to carotid baroceptor stimulation. Microinjections of either (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 50 mM) or (RS)-alpha-cyclopropyl-4-phosphono-phenyl-glycine (CPPG; 80 mM) did not alter baroreflex responses. AIDA blocked group I and III while CPPG blocked all three groups of metabotropic glutamate receptors (mGLURs). These results suggest that ionotropic glutamate receptors, but not mGLURs, in the mNTS mediate the reflex depressor responses to carotid baroreceptor stimulation in the rat.

Oestrogen-dependent tracing in the rat CNS after pseudorabies virus infection

This study examines the hypothesis that neuronal infectivity and the spreading of the pseudorabies virus (PRV) through the synapses in the central nervous system (CNS) are influenced by the oestrogen levels. The arcuate nucleus (ARC) and the subfornical organ (SFO) were chosen as models for analysis; the neurons in both structures possess oestrogen receptors and are mutually connected. A genetically engineered pseudorabies virus (Ba-DupLac) was used as a transneuronal tract tracer. This virus is taken up preferably by axon terminals, and transported very specifically through the synapses in a retrograde manner. Ba-DupLac was injected into the ARC of rats, followed by monitoring of the PRV-immunoreactivity (PRV-IR) in the SFO 72 h following inoculation. We found no PRV immunolabelling in the SFO of ovariectomized (OVX) rats, or in those OVX animals that received oestrogen shortly (4 h) before PRV infection (OVX + E 4 h). In contrast, in those OVX animals that received oestrogen 12 h before PRV infection (OVX + E 12 h), and also in intact control animals, PRV-IR was demonstrated in the SFO in all cases. Surprisingly, a reverse labelling was observed in the OVX rats; PRV-IR appeared in the pyriform cortex, whereas PRV-IR could not be detected in the control and OVX + E 12 h animals. As far as we are aware, this is the first study to demonstrate that transneuronal PRV labelling depends on the effects of oestrogen on certain CNS structures and connections.

Effect of N-glycosylation on ligand binding affinity of rat V1a vasopressin receptor

A rat Vla vasopressin (rVla) receptor has two putative N-glycosylation sites at 14th and 27th amino acid asparagine in the extracellular N-terminus. In the present study, we examined the possible roles of N-glycosylation of the N-terminus in the receptor function. Three point mutants for deglycosylated rVla receptor were generated in which the 14th and/or the 27th asparagine was replaced with glutamine, namely N14Q, N27Q, and N14:27Q, each tagged with an enhanced green fluorescent protein (EGFP) at their C-termini, and transfected to COS-7 or HEK292 cells. The two single mutants and a double mutant have progressively smaller molecular mass compared to the wild-type receptor as determined by immunoblot analysis, indicating that the two sites are effectively glycosylated in vivo. The maximal ligand binding capacities of three mutant receptors were comparable to that of wild-type (17.02 +/- 1.32 pmol/g protein) with modest changes in ligand binding affinities: N27Q and N14:27Q had decreased binding affinities compared to N14Q and wild-type receptors. The reduced binding affinities of the deglycosylated mutants are not likely due to the impaired intracellular transport since their traffickings were indistinguishable from one another. Taken together, these results suggest that the N-glycosylation at the two sites of the N-terminus of rV1a receptor minimally affects the surface expression and trafficking of the receptor.

Gastric effects of methylnaltrexone on mu, kappa, and delta opioid agonists induced brainstem unitary responses

In this study, we evaluated the gastric effects of methylnaltrexone, an opioid receptor antagonist that does not cross the blood-brain barrier in vivo, on mu, kappa and delta opioid agonists induced brainstem unitary responses in an in vitro neonatal rat brainstem-gastric preparation. Single units in the medial subnucleus of the nucleus tractus solitarius (NTS), responding to electrical stimulation of subdiaphragmatic vagal fibers, were recorded. Selective opioid receptor agonists and antagonists were applied only to the gastric compartment of the bath chamber and thus, the brainstem functions of the preparation were not affected by the drugs. The peripheral gastric effects of a mu opioid receptor agonist, DAMGO, and a kappa opioid receptor agonist, U-50,488H, were evaluated on 58 tonic units that received the subdiaphragmatic vagal inputs. For approximately 78% of the units observed, DAMGO (1.0 microM) and U-50,488H (1.0 microM) induced a concentration-dependent inhibition of 62.1+/-9.3% (mean +/- SE) and 49.2+/-6.5% of the control level of the NTS neuronal activity, respectively. Methylnaltrexone competitively antagonized the DAMGO-induced brainstem neuronal effects. Methylnaltrexone at an 18.8-fold higher concentration also reversed U-50,488H-induced NTS neuronal responses. Naloxone, a non-selective opioid receptor antagonist, reversed the inhibitory effects of DAMGO and U-50,488H at much lower concentrations (3.8% and 0.5%, respectively) compared to methylnaltrexone. Only 18% of the NTS neurons evaluated showed inhibitory responses to a delta receptor agonist, DPDPE, (19.7+/-5.0% at 10 microM), and this inhibition could not be reversed by methylnaltrexone in the concentration range we tested. In addition, when methylnaltrexone (1.0 microM) alone was applied to the gastric compartment, there was an activation (8.5+/-2.1%) of the NTS neurons receiving subdiaphragmatic vagal inputs, suggesting an endogenous gastric opioid action in the modulation of brainstem neuronal activities.

Fentanyl infusion preserves cerebral blood flow during decreased arterial blood pressure after traumatic brain injury in cats

Hypotension after traumatic brain injury (TBI) has been associated with significant reductions in cerebral blood flow (CBF) in experimental animals. In humans, posttraumatic hypotension is associated with significantly worsened outcome, possibly because of cerebral hypoperfusion. The existence of opioid receptor-mediated cerebrovascular dilatory effects in humans has been theorized. We studied the systemic and cerebral vascular effects of fentanyl after fluid-percussion injury (FPI) TBI in isoflurane-anesthetized cats. In an approved protocol, 17 fasted cats were anesthetized, mechanically ventilated with 1-1.5% isoflurane in 70% N2O/30% O2, and prepared for FPI. Electroencephalogram (EEG) and intracranial pressure (ICP) were monitored. Cerebral blood flow and cardiac output were measured with radiolabelled microspheres. Animals received moderate FPI (2.2 atm) followed by 15 min of stabilization. Cats were then randomized to control (isoflurane anesthesia plus saline placebo) or fentanyl (isoflurane anesthesia plus fentanyl 50 microg x kg(-1) h(-1)) groups. CBF, EEG, and ICP were recorded at baseline (Baseline), 15 min post-FPI (post-FPI), and at 15, 75, and 135 min after beginning fentanyl or saline placebo infusions (INF 15, INF 75, INF 135). EEG, ICP, PaCO2, PaO2, pH, and temperature were similar between groups. Mean arterial pressure was significantly lower than in the control group after fentanyl administration, while total CBF was not significantly different from control values. In a previous study, decreasing MAP to 80 mm Hg after TBI in isoflurane-anesthetized cats resulted in a 30% decrease in CBF. In this study, fentanyl after TBI significantly decreased MAP but not CBF. Fentanyl administration was associated with preservation of CBF despite hypotension. Further research is necessary to evaluate the effects of fentanyl on cerebral autoregulation after TBI.

Detection of pro-opiomelanocortin mRNA in human and rat caudal medulla by RT-PCR

Pro-opiomelanocortin (POMC) mRNA has been localized in the NTS of the rat, but not in the human or other species. Here, we report that RT-PCR amplification of human caudal medulla RNA generated a distinct band on agarose gels corresponding in size and sequence to the predicted 742-bp POMC PCR product. The 742-bp signal was undetectable following amplification of cortex, amygdala or caudate nucleus RNA. An homologous, 678-bp band was amplified from rat caudal medulla and, unexpectedly, from other brain regions. Competitive RT-PCR demonstrated that POMC cDNA from rat cortex, striatum and cerebellum was 17%, 22% and 45% of caudal medulla levels. These data indicate that the POMC gene is expressed in human caudal medulla and suggest that small amounts of POMC mRNA are present in regions other than the hypothalamus and NTS of rat brain.

Glycyl-L-glutamine [beta-endorphin-(30-31)] attenuates hemorrhagic hypotension in conscious rats

The profound hypotension caused by acute hemorrhage is thought to involve opioid peptide neurons. In this study, we tested whether glycyl-L-glutamine [Gly-Gln; beta-endorphin-(30-31)], a nonopioid peptide derived from beta-endorphin processing, prevents the cardiovascular depression induced by hemorrhage in conscious and anesthetized rats. Previously, we found that Gly-Gln inhibits the hypotension and respiratory depression produced by beta-endorphin and morphine but does not affect opioid antinociception. Hemorrhage (2.5 ml/100 g body wt over 20 min) lowered arterial pressure in conscious rats (from 120.1 +/- 2.9 to 56.2 +/- 4.7 mmHg) but did not change heart rate significantly. Intracerebroventricular Gly-Gln (3, 10, or 30 nmol) pretreatment inhibited the fall in arterial pressure and increased heart rate significantly. The response was dose related and was sustained during the 35-min posthemorrhage interval. Pentobarbital sodium anesthesia potentiated the hemodynamic response to hemorrhage and attenuated the effect of Gly-Gln. Gly-Gln (10 or 100 nmol icv) did not influence arterial pressure or heart rate in normotensive rats. These data indicate that Gly-Gln is an effective antagonist of hemorrhagic hypotension.

Fos induction in central structures after afferent renal nerve stimulation

Experiments were done in the conscious and unrestrained rat to identify central structures activated by electrical stimulation of afferent renal nerves (ARN) using the immunohistochemical detection of Fos-like proteins. Fos-labelled neurons were found in a number of forebrain and brainstem structures bilaterally, but with a contralateral predominance. Additionally, Fos-labelled neurons were found in the lower thoracolumbar spinal cord predominantly ipsilateral to the side of ARN stimulation. Within the forebrain, neurons containing Fos-like immunoreactivity after ARN stimulation were primarily found along the outer edge of the rostral organum vasculosum of the laminae terminalis, in the medial regions of the subfornical organ, in the median preoptic nucleus, in the ventral subdivision of the bed nucleus of the stria terminalis, along the lateral part of the central nucleus of the amygdala, throughout the deeper layers of the dysgranular insular cortex, in the parvocellular component of the paraventricular nucleus of the hypothalamus (PVH), and in the paraventricular nucleus of the thalamus. Additionally, a smaller number of Fos-labelled neurons was observed in the supraoptic nucleus, in the magnocellular component of the PVH and along the lateral border of the arcuate nucleus. Within the brainstem, Fos-labelled neurons were found predominantly in the commissural and medial subnuclei of the nucleus of the solitary tract and in the external subnucleus of the lateral parabrachial nucleus. A smaller number were observed near the caudal pole of the locus coeruleus, and scattered throughout the ventrolateral medullary and pontine reticular formation in the regions known to contain the A1, C1 and A5 catecholamine cell groups. The final area observed to contain Fos-labelled neurons in the central nervous system was the thoracolumbar spinal cord (T9-L1) which contained cells in laminae I-V of the dorsal horn ipsilateral to side of stimulation and in the intermediolateral cell column at the same levels bilaterally, but with an ipsilateral predominance. Few, if any Fos-labelled neurons were observed in the same structures of control animals in which the ARN were stimulated, but the renal nerves proximal to the site of stimulation were transected, or in the sham operated animals. These data indicate that ARN information originating in renal receptors is conveyed to a number of central areas known to be involved in the regulation of body fluid balance and arterial pressure, and suggest that this afferent information is an important component of central mechanisms regulating these homeostatic functions.

Brain melanocortin receptors: from cloning to function

The cloning of brain melanocortin (MC) receptors, the mapping of their expression pattern and the identification of MC receptor selective ligands have opened a new avenue towards elucidating the role of the melanocortin system in the brain. MC receptors have now been implicated in melanocortin-induced grooming behavior in rats, in the melanocortin-induced lowering of blood pressure and in the control of weight homeostasis. Functional opioid antagonism and the anti-pyretic and anti-inflammatory effects of melanocortins are probably also mediated via MC receptors. However, the effects of melanocortins on avoidance behavior and the effect of gamma 2-MSH on increasing blood pressure are not mediated via one of the cloned brain MC receptors. The structure of brain MC receptors, their expression pattern, the MC receptor selective ligands and the function of MC receptors are briefly reviewed.

Melanocortin antagonists define two distinct pathways of cardiovascular control by alpha- and gamma-melanocyte-stimulating hormones

Melanocortin peptides and at least two subtypes of melanocortin receptors (MC3-R and MC4-R) are present in brain regions involved in cardiovascular regulation. In urethane-anesthetized rats, unilateral microinjection of alpha-melanocyte-stimulating hormone (MSH) into the medullary dorsal-vagal complex (DVC) causes dose-dependent (125-250 pmol) hypotension and bradycardia, whereas gamma-MSH is less effective. The effects of alpha-MSH are inhibited by microinjection to the same site of the novel MG4-R/MC3-R antagonist SHU9119 (2-100 pmol) but not naloxone (270 pmol), whereas the similar effects of intra-DVC injection of beta-endorphin (1 pmol) are inhibited by naloxone and not by SHU9119. Hypotensive and bradycardic responses to electrical stimulation of the arcuate nucleus also are inhibited by ipsilateral intra-DVC microinjection of SHU9119. gamma-MSH and ACTH(4-10), but not alpha-MSH, elicit dose-dependent (0.1-12.5 nmol) pressor and tachycardic effects, which are much more pronounced after intracarotid than after intravenous administration. The effects of gamma-MSH (1.25 nmol) are not inhibited by the intracarotid injection of SHU9119 (1.25-12.5 nmol) or the novel MC3-R antagonist SHU9005 (1.25-12.5 nmol). We conclude that the hypotension and bradycardia elicited by the release of alpha-MSH from arcuate neurons is mediated by neural melanocortin receptors (MC4-R/MC3-R) located in the DVC, whereas the similar effects of beta-endorphin, a peptide derived from the same precursor, are mediated by opiate receptors at the same site. In contrast, neither MC3-R nor MC4-R is involved in the centrally mediated pressor and tachycardic actions of gamma-MSH, which, likely, are mediated by an as yet unidentified receptor.

Immunolabeling of Mu opioid receptors in the rat nucleus of the solitary tract: extrasynaptic plasmalemmal localization and association with Leu5-enkephalin

Activation of the mu opioid receptor (MOR) by morphine within the caudal nucleus of the solitary tract (NTS) is known to mediate both cardiorespiratory and gastrointestinal responses. Leu5-enkephalin (LE), a potential endogenous ligand for MOR, is also present within neurons in this region. To determine the cellular sites for the visceral effects of MOR ligands, including LE, we used immunogold-silver and immunoperoxidase methods for light and electron microscopic localization of antisera against MOR (carboxyl terminal domain) and LE in the caudal NTS of rat brain. Light microscopy of coronal sections through the NTS at the level of the area postrema showed MOR-like immunoreactivity (MOR-LI) and LE labeling in punctate processes located within the subpostremal, dorsomedial and medial subnuclei. Electron microscopy of sections through the medial NTS at this level showed gold-silver particles identifying MOR-LI prominently distributed to the cytoplasmic side of the plasma membranes of axons and terminals. MOR labeled terminals formed mostly symmetric (inhibitory-type) synapses but sometimes showed multiple asymmetric junctions, characteristic of excitatory visceral afferents. MOR-LI was also present along extrasynaptic plasma membranes of dendrites receiving afferent input from unlabeled and LE-labeled terminals. We conclude that MOR ligands, possibly including LE, can act at extrasynaptic MORs on the plasma membranes of axons and dendrites in the caudal NTS to modulate the presynaptic release and postsynaptic responses of neurons. These are likely to include local inhibitory neurons and both gastric and cardiorespiratory afferents known to terminate in the subnuclei with the most intense MOR-LI.

Arcuate nucleus inputs onto subfornical organ neurons that respond to plasma hypernatremia and angiotensin II

Experiments were done in urethane anesthetized rats to investigate the effect of electrical and glutamate stimulation of arcuate nucleus (Arc) on the discharge rate of subfornical organ (SFO) neurons that responded to either plasma hypernatremia or angiotensin II (ANG II). Extracellular recordings were made from 253 histologically verified single neurons in SFO. Of these, 40.3% (102/253) responded with excitation and 10% (25/253) with inhibition to Arc stimulation. Thirty-five (34.3%) of the units excited by Arc were also excited by intracarotid infusion of hypertonic (0.5 M) NaCl. In addition, 37 (36.3%) of the units excited by Arc were also excited by intracarotid infusion of ANG II. Furthermore, 10 (40.0%) of the units inhibited by Arc were found to be excited by ANG II. None of the units inhibited by Arc stimulation were responsive to plasma hypernatremia. These data indicate that inputs from Arc neurons converge onto SFO neurons that alter their discharge rate during changes in plasma concentration of Na+ or ANG II. These results suggest that Arc may be involved in body fluid balance and circulatory regulation by modulating the activity of SFO neurons that function in the detection of blood-borne signals from the depletion of intra- and extra-cellular fluid volumes.

Neurotensin projections to subfornical organ from arcuate nucleus

Two series of experiments were done in the rat to investigate whether neurons in arcuate nucleus of the hypothalamus (Arc) containing neurotensin (NT)-like immunoreactivity projected to subfornical organ (SFO). In the first series, the anterograde tract-tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was microiontrophoresed into the region of Arc that contains NT neurons. After a 9-12 day survival period the animals were sacrificed and forebrain sections that contained SFO were processed for combined PHA-L and NT immunoreactivity. In the second series of experiments, unilateral or bilateral electrolytic lesions of Arc were made and after a 10-17 day survival period SFO was examined to determine the relative contribution of NT Arc neurons to NT immunoreactivity within SFO. PHA-L labelled fibers with terminal-like boutons were found in SFO primarily on the side ipsilateral to the site of injection. A small number of the PHA-L labelled fibers in the lateral aspect of SFO was also immunoreactive to NT. Unilateral lesions reduced, whereas bilateral lesions of Arc eliminated most of the NT-like immunoreactivity within SFO. These data demonstrate the existence of a direct pathway from Arc to SFO that contains the putative neurotransmitter NT. These results suggest that this pathway may function in the modulation of neural and/or humoral events related to cardiovascular regulation and body fluid homeostasis by influencing the activity of SFO neurons.

Opioidergic receptors in the arcuate nucleus are not involved in the cardiovascular effects of clonidine

The arcuate nucleus is the bed nucleus for the pro-opiomelancortin system of the brain with important connections with other nuclei involved in cardiovascular function. Clonidine has been reported to produce its cardiovascular effects through an interaction with opioid and alpha 2-adrenergic receptors. The present study examined the arcuate nucleus as a site of action of clonidine. Male spontaneously hypertensive rats were anesthetized with pentobarbital and were instrumented for the measurement of blood pressure and heart rate. Cannulae were placed either through the cisterna magna (IC) or in the arcuate nucleus. Administration of clonidine (0.03-3.75 micrograms, IC) produced a dose-dependent hypotension and bradycardia. Pretreatment with naloxone (30 micrograms, IC) prior to clonidine administration resulted in a significant attenuation of both the clonidine-induced hypotension and bradycardia. In contrast, administration of naloxone (100 ng) into the arcuate nucleus prior to the central administration of clonidine did not alter the cardiovascular effects of clonidine. These results support the role of central opioidergic receptors in the cardiovascular effects of clonidine but do not support the arcuate nucleus as the site of action.

Inverse relationship between beta-endorphin immunoreactivity in cerebrospinal fluid and nucleus tractus solitarius in sudden infant death

In nucleus tractus solitarius (NTS) beta-endorphin (BEND) induces bradycardia and respiratory depression which have been reported to precede death in sudden infant death (SID). Of SID victims, 50% have elevated levels of beta-endorphin immunoreactivity (BENDI) in the cerebrospinal fluid (CSF), and 50% had undetectable levels. We therefore investigated the relationship of BENDI in the CSF to BENDI levels in the NTS area. This study included SID victims (CSF from n = 47, brain stem from n = 16), borderline SID victims (CSF and brain stem from n = 2), sudden death in childhood (CSF and brain stem from n = 1), and controls (CSF from n = 32, brain stem from n = 11). BEND in CSF and NTS area, after extraction, was measured by radioimmunoassay. High performance liquid chromatography was used for closer identification of BENDI. We found that the SID victims divided into two subpopulations, one having a relatively high BENDI level in CSF and one having no detectable level (P < 0.01). Furthermore, an inverse relationship was found between BENDI level in CSF and BENDI level in NTS area in the SID victims (P < 0.05). We conclude that increased BENDI level in CSF is associated with low BENDI level in the NTS area in 50% of SID victims. The low BENDI level in the NTS area may be due to increased release of BEND.

Effect of arcuate nucleus activation on neuronal activity in subfornical organ

Experiments were done in urethane anesthetized rats to investigate the effect of activation of the arcuate nucleus (Arc) on the discharge rate of single units in the subfornical organ (SFO). Extracellular recordings were made from 51 spontaneously active neurons histologically verified in the SFO. Of these units, 19 (37%) were inhibited (mean latency 7.3 +/- 1.1 ms) and 14 (28%) were excited (mean latency 12.8 +/- 3.2 ms) during electrical stimulation of the Arc. The remaining 18 units did not alter their firing frequency during stimulation of the Arc. These data suggest that the Arc may be involved in the modulation of neural and humoral events related to body fluid homeostasis and cardiovascular regulation by altering the activity of SFO neurons.

Relationship of Met-enkephalin-like immunoreactivity to vagal afferents and motor dendrites in the nucleus of the solitary tract: a light and electron microscopic dual labeling study

Methionine (Met5)-enkephalin has been implicated in autonomic functions involving vagal reflexes within the nucleus of the solitary tract (NTS). We examined the light and electron microscopic relationships between neurons containing methionine (Met5)-enkephalin-like immunoreactivity (MELI) and vagal afferents and motor dendrites in the rat NTS. A polyclonal antibody raised against Met5-enkephalin and showing maximal cross-reactivity with this peptide was localized by immunoautoradiography. In the same sections, vagal afferents and motor neurons were identified by histochemical detection of anterogradely and retrogradely transported horseradish peroxidase (HRP). By light microscopy, the MELI was detected in perikarya distributed principally in the dorsomedial, intermediate and parasolitary subdivisions of the NTS. These subnuclei as well as medial and commissural divisions of the NTS also showed: (1) aggregates of silver grains thought to overlie terminals containing MELI, and (2) anterogradely transported HRP in varicose processes. Electron microscopic analysis of the dorsomedial NTS at the level of the area postrema established that MELI was detectable in perikarya, dendrites, and axon terminals. Most of the MELI was associated with large dense core vesicles (dcvs). These opioid terminals formed primarily symmetric synapses on proximal and asymmetric synapses on distal dendrites. Analysis of the dendritic targets of terminals containing MELI revealed that 13/222 were in synaptic contact with dendrites also containing MELI. The remainder of the terminals containing MELI either lacked recognized junctions or formed synapses with unlabeled dendrites. In comparison to the terminals containing MELI in the same series of sections, anterogradely labeled vagal terminals extensively formed asymmetric junctions with distal dendrites and spines. Of the observed anterogradely labeled terminals 6/84 formed synapses with dendrites containing MELI and 3/84 with dendrites containing retrogradely transported HRP. The remainder of the junctions were with dendrites lacking detectable immunoautoradiographic or HRP-labeling. The majority of the recognized synapses on labeled dendrites were at more proximal sites possibly reflecting more limited detection of both MELI and retrogradely transported HRP in smaller dendrites. However, the presence of even a few junctions at proximal sites on dendrites where synaptic transmission is known to be more effective suggests a potentially strong modulation of both opioid and vagal motor neurons by visceral afferents in the NTS. In addition to forming synapses on dendrites, both vagal afferents and terminals containing MELI showed frequent synaptic associations with unlabeled terminals, but not with each other. This finding suggests that the previously demonstrated opiate binding sites on vagal afferents is most likely attributed to other endogenous opiates.

Organization of peptidergic and catecholaminergic efferents from the nucleus of the solitary tract to the rat amygdala

Previous studies have focused on the role of the central nucleus of the amygdala (CeA) in cardiovascular and other amygdaloid functions. The combined retrograde tracing/immunohistochemical method was used to test for the presence of enkephalin, neurotensin, neuropeptide Y, and catecholamine neurons within the nucleus of the solitary tract that send efferents to the CeA. After injections of retrograde tracer into the CeA, retrogradely labeled neurons were observed within the caudal, medial nucleus of the solitary tract. Most CeA-projecting neurons were located ipsilaterally within the medial nucleus of the solitary tract at the level of the area postrema. Retrogradely labeled enkephalin- and neurotensin-immunoreactive neurons were found within the medial nucleus of the solitary tract at this level, while retrogradely labeled neuropeptide Y-immunoreactive neurons were found within the medial nucleus of the solitary tract rostral to the area postrema. About 60-74% of CeA-projecting cells were also immunoreactive for tyrosine hydroxylase. Approximately 9% of retrogradely neurons were phenylethanolamine-N-methyltransferase immunoreactive. The results provide evidence that within the nucleus of the solitary tract, peptidergic CeA-projecting neurons have a topographic distribution. In addition, noradrenergic neurons within the A2 group, rather than adrenergic neurons of the C2 group, provide the bulk of catecholaminergic input to the CeA from the nucleus of the solitary tract. Cell counts indicate that each of these peptides may be colocalized (to varying extents) within catecholamine-producing neurons. Also the catecholaminergic and enkephalinergic contribution to the ascending pathway from the nucleus of the solitary tract to the CeA distinguishes it neurochemically from the descending pathway. Thus, although there are afferent and efferent connections between the nucleus of the solitary tract and CeA, their peptidergic/neurotransmitter connections are not necessarily reciprocal. Input from nucleus of the solitary tract peptidergic and catecholaminergic neurons to the CeA may be important in the etiology of a number of pathophysiological conditions including hypertension, gastric ulcers, and schizophrenia.

Ethanol inhibition of baroreflex bradycardia: role of brainstem GABA receptors

Ethanol administered i.v. or into the nucleus tractus solitarii (NTS) of rats anaesthetized with urethane inhibits baroreflex bradycardia elicited by phenylephrine. This effect is prevented or reduced by pretreatment of rats with 3-mercaptopropionic acid, bicuculline, or RO 15-4513. Intra-NTS injection of muscimol also inhibits baroreflex bradycardia and causes a pressor response which is potentiated by intra-NTS ethanol. It is proposed that ethanol inhibits baroreflex bradycardia, at least in part, by potentiating the action of endogenous gamma-aminobutyric acid (GABA) at GABAA receptors in the NTS or its vicinity.